Refactor binding.DataRoot

Refactor binding.DataRoot

DataRoot is really a (currently unaddressable) DataContainer. Make it
generic, forcing specialization, and extend DataContainer.

This ends up cleaning up some sites, which are using generic capture
<R extends DataRoot & DataObject> to express a generated type -- those
now become <R extends DataRoot<R>>.

We also get the goodies of terminal specialization like OpaqueObject,
like an accurate implementedInterface().

JIRA: YANGTOOLS-1577
Change-Id: Ie37fad4b7274e06dbf412c6d3a1cee3d6a6c98c0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>